Clinical neurological methods in the diagnosis of the hand-arm vibration syndrome.

FARKKILA M. Clinical neurologicalmethodsin the diagnosisof the hand-arm vibration syndrome. Scand J Work Environ Health 13(1987) 367- 369. Methods for assessingneurological disturbancesin thediagnosis of the hand-arm vibration syndrome are described. The clinical examination should comprise a careful historyof symptomsand vibrationexposureand a considerationof neurologicalsignsknownto beassociated with vibration exposure. There is as yet no etiologicdiagnostic method for vibration-induced neurological lesions. The bestmethod isa relevant clinicalexaminationas described, in combinationwitha quantitative assessmentof sensory modalities(suchas thevibration perceptionthreshold)and an electroneuromyographic test.

The mo st pr om inent sign of the hand-arm vibr at ion syndro me is the vascular disorder prev iously known as "traumatic vasospastic disease, " now called Raynaud 's ph enom enon or vibra tion-i nd uced white finger (VW F) (2,22,26). Th e vascular sympto ms a ppear to be cau sed both by central sympa thetic mech anisms and by a peripheral local lesio n in the co ntrol of blood flow in the fing ers (20,23). In spite of int ensive study, the ba sic pathogen ic mechanism of VWF is still unknown . A s di fferent mechani sms may underlie th e vasc ular and senso rineura l components of VWF, separate clinical classification scales for the two type s o f sympto ms ha ve been proposed (5,13).
Neu rological disturban ces other th an th ose occurring in connection with VWF have been des cribed for vibration-expos ed workers (diffuse neuropathy, carpal tunnel synd rome, etc). Th e methods suggested for use in the diagnosis o f th ese disorder s are as foll ows: (i) clinica l neurological examination, (ii) electroneuromyography, (iii) assessment of somatosensory evok ed potent ials, (iv) quantitative mea surement of senso ry modalities, and (v) mu scle for ce measurement. Th ey are all nonspecific with regard to vibration as a n etiolo gic fa ctor.
The best method for diagnosing the neurological disturban ces co nnected wit h the hand-arm vibratio n syndrome is still a clinical neurological examination with due consideration o f sympto m history, vibra tionexposure data, and differential etiology. In a larg e population study (10)

Electroneuromyography
The electroneuromyogram has been exten sively used for th e dia gnosis of vibration-induced neuropathy.
Electroneuromyography may indicate the presence and location of a peripheral ner ve lesion . Lu kas (17) found electroneuromyographic changes in 53°10 of 108 wo rkers expo sed to vibra tion. These findings were interpreted as possible con sequences of spinal disease of the neck, compression in the upper thoracic aperture, disease affecting the elbow joint, and disturbances in the ulnar or median nerves in the hand. In general, the findings were not taken to be causally related to vib ration exposure. Lukas could not state unequivocally that expo sure to vibration was the primary cause of the changes he detected, no r could he exclude th is po ssibilit y. Changes in the distal latencies of the median nerve ha ve been described to be specific to vibration neuropathy (24), especially in combination with ulnar nerve a ffection. However , the po ssible intraneural entrap-ment induced by vibration exposure -first described by Ahlborg & Voog (4) -probably causes this condition, which resembles carpal tunnel syndrome (21). Electroneuromyography is a very sensitive method for demonstrating nerve lesion; the real problem, however, is to show that vibration has caused the lesion.

Assessment of somatosensory evoked potentials
In clinical practice, the assessment of somatosensory evoked potentials has been recently introduced as a routine method for the study of both the peripheral sensory nerves and the central neural pathways. It appears that this assessment has not yet been used in clinical work for the diagnosis of neurological disturbances in the hand-arm vibration syndrome. Theoretically, it may offer a new way of approaching these nerve lesions. However, sensitive as this method is, it will probably prove to be even more vibration nonspecific than electroneuromyography.
The quantitative measurement of sensory modalities, chiefly the sense of vibration, has been extensively tried in the diagnosis of vibration syndrome (1,19,25). The two-point discrimination test and the assessment of temperature sensitivity have also been used. These tests are psychophysical, and variations in the degree of cooperation of the subject may greatly affect the results. Another problem is that, although the tests are sensitive enough, they normally show a large interindividual variation and therefore cannot be used for individual diagnostics (12). They may, however, be used with advantage in group research. A modified method for the assessment of vibration perception threshold has been developed (8) in which the adaptation of the receptors after vascular occlusion is recorded. The adaptation curve of the vibration perception threshold seems to differentiate better between persons with VWF and those with carpal tunnel syndrome. However, this test, like several of its kind, totally lacks etiologic specificity.

Muscle force measurement
Whether changes in muscle force form part of the hand-arm vibration syndrome is a question that has been controversial for a long time. Agate and his collaborators (3) rejected this idea, and later work (15) has supported their opinion. Lumberjacks with only VWF have been found to show a higher level of muscle force than lumberjacks with so-called vibration neuropathy (7,9). There is, however, no range of hand muscle force which is known to be "normal." Therefore, muscle force measurements can only be used in group studies, not for individual diagnostics. It should also be remembered that the assessment of muscle force depends to a certain degree on subjective factors.

Differential diagnostic aspects in neurological problems
In a broad reviewof polyneuropathies, Freemon (11)did not mention vibration as a cause of generalized nerve lesions. Muscle weakness, numbness, and clumsiness of the hands in persons exposed to hand-arm vibration have been considered, for a long time, as indicating vibration neuropathy (16,18,24). Differential diagnostic problems arise from alcohol consumption, systemic diseases like diabetes, collagenoses, thoracic outlet syndrome, carpal tunnel entrapment, deficiency of vitamins, previous injuries to the hand and arm, etc. Recently, a case report of primary amyloidosis in a vibration-exposed worker has been published (14). In insulin-dependent diabetic patients followed for many years during their vibration work, no signs of polyneuropathy have been seen to develop. In a comparison of teetotallers with heavy alcohol consumers, no relationship was observed between alcohol consumption and symptoms and signs of neuropathy (10).
Carpal tunnel syndrome has been reported to occur more often among vibration-exposed workers than in a normal male population (4,6). The clinical picture of carpal tunnel syndrome that is induced by vibration slightly differs from carpal tunnel entrapment of other origin. The entrapment symptoms in the syndrome are more diffuse, often affecting also the ulnar nerve. It shows good spontaneous recovery after vibration exposure is eliminated. Surgical therapy is seldom needed because of this tendency towards spontaneous recovery (8). At present, it seems that vibration exposure to the hands can cause connective tissue swelling, especially in the carpal canal, and diffuse symptoms of entrapment, particularly of the median nerve. This phenomenon explains both the hand paresthesias and the neurophysiological findings already discussed. Such a condition can develop independently of VWF and should be included as a separate entity of the hand-arm vibration syndrome.